res_ninit, res_nquery, res_nsearch, res_nquerydomain, res_nmkquery, res_nsend, res_init, res_query, res_search, res_querydomain, res_mkquery, res_send, dn_comp, dn_expand — resolver routines


#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

struct __res_state;
typedef struct __res_state *res_state;
int res_ninit( void);  
int res_nquery( res_state statep,
  const char *dname,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_nsearch( res_state statep,
  const char *dname,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_nquerydomain( res_state statep,
  const char *name,
  const char *domain,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_nmkquery( res_state statep,
  int op,
  const char *dname,
  int class,
  int type,
  const unsigned char *data,
  int datalen,
  const unsigned char *newrr,
  unsigned char *buf,
  int buflen);
int res_nsend( res_state statep,
  const unsigned char *msg,
  int msglen,
  unsigned char *answer,
  int anslen);
int dn_comp( const char *exp_dn,
  unsigned char *comp_dn,
  int length,
  unsigned char **dnptrs,
  unsigned char **lastdnptr);
int dn_expand( const unsigned char *msg,
  const unsigned char *eomorig,
  const unsigned char *comp_dn,
  char *exp_dn,
  int length);


extern struct __res_state _res;
int res_init( void);  
int res_query( const char *dname,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_search( const char *dname,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_querydomain( const char *name,
  const char *domain,
  int class,
  int type,
  unsigned char *answer,
  int anslen);
int res_mkquery( int op,
  const char *dname,
  int class,
  int type,
  const unsigned char *data,
  int datalen,
  const unsigned char *newrr,
  unsigned char *buf,
  int buflen);
int res_send( const unsigned char *msg,
  int msglen,
  unsigned char *answer,
  int anslen);
[Note] Note

Link with −lresolv.


[Note] Note

This page is incomplete (various resolver functions provided by glibc are not described) and likely out of date.

The functions described below make queries to and interpret the responses from Internet domain name servers.

The API consists of a set of more modern, reentrant functions and an older set of nonreentrant functions that have been superseded. The traditional resolver interfaces such as res_init() and res_query() use some static (global) state stored in the _res structure, rendering these functions non-thread-safe. BIND 8.2 introduced a set of new interfaces res_ninit(), res_nquery(), and so on, which take a res_state as their first argument, so you can use a per-thread resolver state.

The res_ninit() and res_init() functions read the configuration files (see resolv.conf(5)) to get the default domain name and name server address(es). If no server is given, the local host is tried. If no domain is given, that associated with the local host is used. It can be overridden with the environment variable LOCALDOMAIN. res_ninit() or res_init() is normally executed by the first call to one of the other functions.

The res_nquery() and res_query() functions query the name server for the fully qualified domain name name of specified type and class. The reply is left in the buffer answer of length anslen supplied by the caller.

The res_nsearch() and res_search() functions make a query and waits for the response like res_nquery() and res_query(), but in addition they implement the default and search rules controlled by RES_DEFNAMES and RES_DNSRCH (see description of _res options below).

The res_nquerydomain() and res_querydomain() functions make a query using res_nquery()/res_query() on the concatenation of name and domain.

The following functions are lower-level routines used by res_query()/res_query().

The res_nmkquery() and res_mkquery() functions construct a query message in buf of length buflen for the domain name dname. The query type op is usually QUERY, but can be any of the types defined in <arpa/nameser.h> newrr is currently unused.

The res_nsend() and res_send() function send a preformatted query given in msg of length msglen and returns the answer in answer which is of length anslen. They will call res_ninit()/res_init() if it has not already been called.

The dn_comp() function compresses the domain name exp_dn and stores it in the buffer comp_dn of length length. The compression uses an array of pointers dnptrs to previously compressed names in the current message. The first pointer points to the beginning of the message and the list ends with NULL. The limit of the array is specified by lastdnptr. If dnptr is NULL, domain names are not compressed. If lastdnptr is NULL, the list of labels is not updated.

The dn_expand() function expands the compressed domain name comp_dn to a full domain name, which is placed in the buffer exp_dn of size length. The compressed name is contained in a query or reply message, and msg points to the beginning of the message.

The resolver routines use configuration and state information contained in a __res_state structure (either passed as the statep argument, or in the global variable _res, in the case of the older nonreentrant functions). The only field of this structure that is normally manipulated by the user is the options field. This field can contain the bitwise "OR" of the following options:


True if res_ninit() or res_init() has been called.


Print debugging messages. This option is available only if glibc was built with debugging enabled, which is not the default.


Accept authoritative answers only. res_send() continues until it finds an authoritative answer or returns an error. [Not currently implemented].


Use TCP connections for queries rather than UDP datagrams.


Query primary domain name server only. [Not currently implemented].


Ignore truncation errors. Don't retry with TCP.


Set the recursion desired bit in queries. Recursion is carried out by the domain name server, not by res_send(). [Enabled by default].


If set, res_search() will append the default domain name to single component names—that is, those that do not contain a dot. [Enabled by default].


Used with RES_USEVC to keep the TCP connection open between queries.


If set, res_search() will search for hostnames in the current domain and in parent domains. This option is used by gethostbyname(3). [Enabled by default].


Accept a response from a wrong server. This can be used to detect potential security hazards, but you need to compile glibc with debugging enabled and use RES_DEBUG option (for debug purpose only).


Accept a response which contains a wrong query. This can be used to detect potential security hazards, but you need to compile glibc with debugging enabled and use RES_DEBUG option (for debug purpose only).


Disable usage of HOSTALIASES environment variable.


Try an AAAA query before an A query inside the gethostbyname() function, and map IPv4 responses in IPv6 "tunneled form" if no AAAA records are found but an A record set exists.


Causes round-robin selection of name servers from among those listed. This has the effect of spreading the query load among all listed servers, rather than having all clients try the first listed server first every time.


Disable the modern BIND checking of incoming hostnames and mail names for invalid characters such as underscore (_), non-ASCII, or control characters. [Not currently implemented].


Do not strip TSIG records. [Not currently implemented].


Send each query simultaneously and recursively to all servers. Note this option overrides RES_ROTATE.

RES_USEBSTRING (since glibc 2.3.4)

Make reverse IPv6 lookups using the bit-label format described in RFC 2673; if this option is not set, then nibble format is used.


Use zone in IPv6 reverse lookup instead of, which is deprecated since glibc 2.3.4. [Enabled by default].

RES_USE_EDNS0 (since glibc 2.6)

Enables support for the DNS extensions (EDNS0) described in RFC 2671.

RES_SNGLKUP (since glibc 2.10)

By default, glibc performs IPv4 and IPv6 lookups in parallel since version 2.9. Some appliance DNS servers cannot handle these queries properly and make the requests time out. This option disables the behavior and makes glibc perform the IPv6 and IPv4 requests sequentially (at the cost of some slowdown of the resolving process).


When RES_SNGLKUP option is enabled, opens a new socket for the each request.


Use DNSSEC with OK bit in OPT record. This option implies RES_USE_EDNS0.


Do not look up unqualified name as a top-level domain (TLD).


Default option which implies: RES_RECURSE, RES_DEFNAMES, RES_DNSRCH and RES_NOIP6DOTINT.


The res_ninit() and res_init() functions return 0 on success, or −1 if an error occurs.

The res_nquery(), res_query(), res_nsearch(), res_search(), res_nquerydomain(), res_querydomain(), res_nmkquery(), res_mkquery(), res_nsend(), and res_send() functions return the length of the response, or −1 if an error occurs.

The dn_comp() and dn_expand() functions return the length of the compressed name, or −1 if an error occurs.


/etc/resolv.conf          resolver configuration file
/etc/host.conf            resolver configuration file


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value

res_ninit(), res_nquery(),

res_nsearch(), res_nquerydomain(), res_nsend()

Thread safety MT-Safe locale

res_nmkquery(), dn_comp(),


Thread safety MT-Safe




gethostbyname(3), resolv.conf(5), resolver(5), hostname(7), named(8)

The GNU C library source file resolv/README.


This page is part of release 4.07 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright 1993 David Metcalfe (
and (C) Copyright 2015 Michael Kerrisk <>

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manual provided the copyright notice and this permission notice are
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References consulted:
    Linux libc source code
    Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
    386BSD man pages
Modified 1993-07-25 by Rik Faith (
Modified 2004-10-31 by aeb